System and method for stall detection of a motor

ABSTRACT

A motor starter system comprises solid state switches for connection between an AC line and motor terminals for controlling application of AC power to the motor. A voltage sensor senses AC line voltage and the motor terminal voltage. Current sensors sense motor current. A control circuit controls operation of the solid state switches. The control circuit limits switch current during a start mode and detects a stall condition responsive to sensed AC line voltage and motor terminal voltage and selectively boosts motor current during the start mode if a stall condition is detected.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

[0001] This application claims priority of Provisional Application No.60/446,940, filed Feb. 12, 2003.

FIELD OF THE INVENTION

[0002] This invention relates to a motor controller and moreparticularly, a system and method for stall detection of a motor.

BACKGROUND OF THE INVENTION

[0003] Solid state starters/controllers have found widespread use forcontrolling application of power to an AC induction motor. Theconventional starter/controller, referred to hereinafter as simply astarter or a controller, uses solid state switches for controllingapplication of AC line voltage to the motor. The switches may bethyristors such as silicon controlled rectifiers (SCRs) or triacs.

[0004] One application for a motor controller is as an elevator starter.The elevator starter may be used to drive a pump for an hydraulicelevator. Each time movement of an elevator car is commanded, then thestarter must start the motor until it reaches operating speed and thenoperate in a run mode. Such a starter may only be used for the updirection as gravity may be used for the down direction.

[0005] One type of elevator starter initially connects the motorwindings in a Y-configuration to start the motor and bring it up tospeed. Then the windings are reconnected in a delta configuration withfull voltage. Other starters, referred to as soft starters, change theon time of the solid state switches to control voltage and to ramp upmotor current with a fixed connection. Known elevator starters haveselector switches for setting a starting current limit setting.Depending on configuration, the setting is adjustable from about 100percent to 450 percent of the starter's current rating. The timerequired to bring a motor up to speed with a current limit start is afunction of the difference in the torque provided at the current limitsetting and the torque required to accelerate the load or pump. As ageneral rule, the higher the current limit setting the lower the starttime and conversely, the lower the current limit setting the longer thestart time. In an elevator application end users are interested instarting the motor as fast as possible.

[0006] In applications where the load during the start is light anddoesn't vary from start to start, for example a hydraulic elevatormotor, current limit is generally the preferred method to start themotor. However, if the torque required to start the load or pumpincreases, then the start time will increase. Depending on the torquerequired to start the load or pump, it may take an unsuitably long timeto get the motor up to speed. To compensate for this time delay, knownsoft starters increase the motor start current past the current limitsetting if the motor does not come up to speed in an allotted time. Somestarters use a fixed time or a variable time based on the average starttime. The delay time enables the rotor to accelerate up to theappropriate speed before additional current boost is given under normalconditions. This routine works well when the torque is sufficient toallow the rotor to continue to accelerate during the allotted timeframe.

[0007] If the load is not up to speed in the allotted time, then theboost provided when the starter increases the current typically bringsthe rotor up to speed. However, there can be exceptions that result infailure of the motor start operation.

[0008] As long as the torque provided by the current limit settingremains higher than the torque required to accelerate the load or pumpthroughout the torque curve, the motor will continue to accelerate untilit increases to an appropriate speed. If at any time during the startoperation the torque provided by the current limit setting is equal tothe torque required to accelerate the load, then the motor will nolonger be able to accelerate and will remain at constant speed until thecurrent boost algorithm is implemented. During this time the motor willcontinue to spin at the “stalled” speed. In applications where thestarting current is around 200% of the motor's full load current, theaverage start time may exceed 2 to 2.5 seconds. This can result in adelay of up to 5 seconds before the starter begins to boost the currentin an effort to bring the motor up to speed. The time spent in the delayis wasted as the motor is spinning at the same low speed when the timedelay expires as it was when the stalled condition was encountered,regardless of the length of the delay.

[0009] The present invention is directed to solving one or more of theproblems discussed above, in a novel and simple manner.

SUMMARY OF THE INVENTION

[0010] In accordance with the invention there is provided a system andmethod for stall detection of a motor.

[0011] Broadly, there is disclosed herein a motor controller systemcomprising solid state switches for connection between an AC line andmotor terminals for controlling application of AC power to the motor. Acurrent sensor senses motor current. A voltage sensor senses voltage. Acontrol circuit controls operation of the solid state switches. Thecontrol circuit limits switch current during a start mode and detects astall condition responsive to sensed voltage and selectively boostsmotor current during the start mode if a stall condition is detected.

[0012] It is a feature of the invention that the voltage sensor sensesRMS voltage.

[0013] It is another feature of the invention that the voltage sensorsenses instantaneous voltage for each winding of the motor anddetermines RMS motor voltage for each winding. The control circuitdetects a stall condition using an average of the RMS motor voltage foreach winding.

[0014] It is another feature of the invention that the control circuitvaries firing of the solid state switches to control motor startingcurrent.

[0015] It is an additional feature of the invention that the controlcircuit detects a stall condition if a difference in the sensed voltageafter a select interval is less than a select threshold. The selectinterval is about ten electrical cycles.

[0016] There is disclosed in accordance with another aspect of theinvention a motor starter system comprising solid state switches forconnection between an AC line and motor terminals for controllingapplication of AC power to the motor. A voltage sensor senses AC linevoltage and the motor terminal voltage. Current sensors sense motorcurrent. A control circuit controls operation of the solid stateswitches. The control circuit limits switch current during a start modeand detects a stall condition responsive to sensed AC line voltage andmotor terminal voltage and selectively boosts motor current during thestart mode if a stall condition is detected.

[0017] There is disclosed in accordance with yet another aspect of theinvention a motor starter system comprising solid state switch means forconnection between an AC line and motor terminals for controllingapplication of AC power to the motor. Voltage sensing means sense motorwinding voltage. Current sensing means sense motor current. Startcontrol means are operatively connected to the voltage sensing means andthe current sensing means for controlling operation of the solid stateswitch means during a start mode. The start control means limits switchcurrent during the start mode and detects a stall condition responsiveto sensed motor winding voltage and selectively boosts motor currentduring the start mode if a stall condition is detected.

[0018] There is disclosed in accordance with still a further aspect ofthe invention a method of detecting a stall condition during motorstarting comprising: providing solid state switches for connectionbetween an AC line and motor terminals for controlling application of ACpower to the motor; sensing motor voltage; sensing motor current;controlling operation of the solid state switches during a start mode tolimit switch current during the start mode; detecting a stall conditionresponsive to sensed motor voltage; and boosting motor current duringthe start mode if a stall condition is detected.

[0019] Further features and advantages of the invention will be readilyapparent from the specification and from the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a perspective view of a motor controller in accordancewith the invention;

[0021]FIG. 2 is a block diagram of the motor controller of FIG. 1;

[0022]FIG. 3 is a wiring diagram of the motor controller of FIG. 1connected to a motor in a delta configuration;

[0023]FIG. 4 is a curve illustrating current control for the SCRs of themotor controller;

[0024]FIG. 5 is a flow diagram illustrating a stall detection moduleimplemented by the processor of FIG. 2; and

[0025]FIG. 6 is a wiring diagram for an alternative connection of themotor controller in line with a motor.

DETAILED DESCRIPTION OF THE INVENTION

[0026] Referring initially to FIG. 1, a solid state motorstarter/controller 20, referred to hereinafter as simply a starter or acontroller, is illustrated. One application for the controller 20 is asan elevator starter. The motor controller 20 may be used to drive a pumpfor an hydraulic elevator. Each time movement of an elevator car iscommanded, the motor controller 20 must start the elevator motor untilit reaches operating speed and then operate in a run mode. Such a motorcontroller 20 may only be used for the up direction as gravity may beused for the down direction.

[0027] The motor controller 20 comprises a housing 22 including ahousing base 24, a heat sink 26 and a cover 28. The motor controller 20includes a plurality of solid state switches 32 in the form ofthyristors, such as back to back connected silicon controlled rectifier(SCR) pairs, see FIG. 2. For simplicity herein, the SCR pairs 32 arereferred to as simply SCRs. Triacs could also be used. The SCRs 32control application of three phase AC line voltage to a three phasemotor. As is apparent, a different number of SCRs 32 could be used tocontrol different numbers of phases, as is apparent to those skilled inthe art.

[0028] The SCRs 32 are mounted to the heat sink 26 within the housing20. Referring also to FIG. 2, a control circuit 34 is also enclosed inthe housing 20. The control circuit 34 controls operation of the SCRs32. Particularly, the control circuit 34 includes a programmed processor36, such as a digital signal processor, for commanding operation of theSCRs 32. A memory 38 is connected to the processor 36 and storesprograms and configuration information relating to operation of the SCRs32, as described below. As is apparent, the processor 36 may includeprogram memory storing some or all of the programs and configurationinformation.

[0029] The processor 36 is connected to three interface circuits 40 eachfor connection to one of the SCRs 32. Particularly, the interfacecircuits 40 comprise snubber circuits for driving the SCRs 32 andvoltage sense circuits for sensing line voltage and motor terminalvoltage. A current transformer 42 senses current of each of the SCRs 32and is connected to a current sense circuit 44. Other types of currentsensors could be used. The current sense circuit 44 is also connected tothe processor 36.

[0030] An LCD display 45 on the cover 22, see FIG. 1, is connected tothe processor 36. The display 45 is used to indicate configurationsettings, operating values, fault conditions, and the like. Useractuable switches 46 are electrically connected to the processor 36. Theuser actuable switches 46 are actuated by actuator elements 48 on thehousing cover 22, see FIG. 1. Particularly, the switches 46 are used forlocally selecting parameters for stored configuration information.

[0031] Referring to FIG. 3, an electrical schematic illustratesconnection of the SCRs 32 of FIG. 2 to motor windings in a typical deltaconfiguration. The terminals of the motor are referenced by a “t”. Allterminals of the controller 20 are referenced by “T” and “L”designations. Thus, each SCR 32 is connected between a pair ofcontroller terminals, such as the terminals L1 and T1. For example, oneof the SCRs 32 is connected between the first phase line voltage L1 andthe first motor terminal t1. The first motor winding W1 is connected inseries with the SCR 32 between the motor terminal t1 and another motorterminal t4. A fault contact FC1 is also connected in series. The otherlegs of the delta configuration are generally similar and areconventional in nature. As is apparent, other motor configurations couldbe used in connection with the disclosed system and method.

[0032] The processor 36 of FIG. 2 operates in accordance with a controlprogram for controlling operation of the SCRs 32. Particularly, each SCR32 is conventionally controlled to satisfy voltage and currentrequirements. This is done by altering the firing angle of the SCRs 32.FIG. 4 shows a graphical illustration including a line curve 50representing input current. A vertical arrow 52 represents firing angleof the SCRs 32. As is conventional, the firing angle 52 is controlled bythe processor 36 to satisfy operating requirements. To lower current thefiring angle 52 would be moved to the right in FIG. 4 to decreaseconduction time. Conversely, to increase current the firing angle 52would be moved to the left to increase conduction time, as is wellknown. During start mode, the processor 36 ramps the current up bygradually advancing the firing angle 52 in a time specified to satisfypreselect acceleration time and acceleration torque values up to aselect starting current limit setting value. By adjusting the delay infiring the SCRs 32, the processor 36 can maintain this level. As themotor speed increases, the current begins to decrease. The processor 36continually increases the voltage to offset the reduction in current.This maintains a constant current at the setting of the starting currentlimit switch. Subsequently during a run mode the control circuit 34applies full voltage to the motor.

[0033] In accordance with the invention, the control program implementsa novel system and method of immediately recognizing a stall conditionand implementing a boost algorithm without waiting for a delay totranspire.

[0034] There are several different methods that could be used to senseif a motor has entered a stall condition during the start and is nolonger accelerating. One solution is to monitor the time between a zerocrossing of line voltage and the firing of the SCR. This time period isreferred to as the “off delay”. If the off delay does not change whilethe sensed current remains constant during the start mode, it can beassumed that the motor is no longer accelerating and is in a stallcondition. However, if the incoming line voltage fluctuates during thestart, the off delay will not remain constant. Two examples ofsituations where the incoming voltages may fluctuate are brown outconditions and when operating on generator power.

[0035] Another solution is to monitor the actual motor voltage and motorcurrent. If at a constant current the motor voltage is not changing,then the motor is in a stall condition. Monitoring motor voltageeliminates the variances associated with the off delay method as thestarter continually adjusts the voltage going to the motor to keep thecurrent at the desired setting. If a voltage dips, such as during abrown out condition, then the off delay will decrease as the startermaintains the motor voltage. If a generator increases the voltage as itbegins to recover from an increased load, then the starter may actuallyincrease the firing delay. In either instance with a stall condition,the motor voltage will remain approximately equal.

[0036] In accordance with the invention, the control program comparesthe average of the three RMS motor winding voltages at a predeterminedinterval to determine if a motor is in a stall condition. If thedifference between two consecutive sensed voltages is less than apredetermined amount, then it is determined that the motor is in a stallcondition and the current boost is immediately implemented.

[0037] As discussed above, the control circuit 34 senses motor currentusing the current sensors 42 and the current sense circuit 44. Theinterface circuits 40 sense the voltages at the SCR terminals L1, L2 andL3 on the line side and the terminals T1, T2 and T3 on the motor side.By taking the difference between the T1 voltage and the L2 voltage whenthe fault contactor is closed, the instantaneous voltage can be derivedfor the first winding W1 connected between the motor terminals t1 andt4. Likewise, the instantaneous voltage for the second winding W2 can bederived by looking at the T2 and L3 voltages and the instantaneousvoltage for the third winding W3 can be derived by looking at thevoltage between the terminals T3 and L1. The instantaneous voltage foreach winding W1-W3 is used to calculate the RMS voltage for eachwinding. The average of the RMS voltages for the three windings W1-W3 isused to detect a stall condition.

[0038] In accordance with the invention, the control circuit 34 utilizesa stall detection module 54, see FIG. 2, to immediately recognize astall condition and implement a boost current algorithm during a startmode.

[0039] Referring to FIG. 5, a flow diagram illustrates the program forthe stall detection module 54. The module 54 is implemented during eachtime starting operation is commanded by the processor 36. As will beapparent, the motor start operation includes conventional programmingfor selecting current limit values and a ramping operation. Thoseprograms are conventional in nature and are not discussed in detailherein.

[0040] The module 54 begins at a decision block 60 which determines ifthe current sensed by the current sense circuit 44, representing motorcurrent, is less than a commanded starting current. If the sensedcurrent is less than the commanded starting current, then a block 62increases firing, as discussed above relative to FIG. 4, to increasecurrent. If the sensed current is equal to the commanded startingcurrent, then the firing is maintained at a block 64. If the sensedcurrent is greater than the commanded starting current, then firing isdecreased at a block 66 to lower current, again as discussed aboverelative to FIG. 4.

[0041] From any of the blocks 62, 64 or 66, a decision block 68determines if a select interval of time has passed since the last motorvoltage comparison was made. In the illustrated embodiment of theinvention, the select interval is ten electrical cycles. As is apparent,the interval could be a different interval or based on a set time, aswill be apparent to those skilled in the art. If ten electrical cycleshave not passed, then control returns to the decision block 60. If tenelectrical cycles have passed, then a decision block 70 determines ifthere has been a voltage change. Particularly, the module 54 determinesif the current value of the average RMS motor voltage, discussed above,is greater than the most recent value of the average RMS voltage by aselect amount. If so, then there is no stall condition and the modulereturns to the decision block 60. If the voltage has not changed by theselect threshold amount, then the module 54 detects a stall condition.In response to a stall condition, a conventional boost current algorithmis implemented at a block 72. For example, the boost current algorithm72 may increase the current limit to 450% of rated current to bring therotor up to speed.

[0042] Referring to FIG. 6, an electrical schematic illustrates aportion of the control circuit 34 for connecting the starter in linewith a motor M. In this application, the instantaneous motor windingvoltages are taken from terminals T1 to T2, from T2 to T3, and from T3to T1, to determine the RMS motor voltages in combined t1, t2 and t3winding connections. An average of these voltages is used for the motorvoltage calculation.

[0043] It can therefore be appreciated that a new and novel system andmethod for automatically detecting a stall condition and boosting startcurrent in a motor controller has been described. It will be appreciatedby those skilled in the art that, given the teaching herein, numerousalternatives and equivalents will be seen to exist which incorporate thedisclosed invention. As a result, the invention is not to be limited bythe foregoing exemplary embodiments, but only by the following claims.

We claim:
 1. A motor controller system comprising: solid state switchesfor connection between an AC line and motor terminals for controllingapplication of AC power to the motor; current sensors for sensing motorcurrent; a voltage sensor for sensing voltage; and a control circuit forcontrolling operation of the solid state switches, the control circuitlimiting switch current during a start mode and detecting a stallcondition responsive to sensed voltage and selectively boosting motorcurrent during the start mode if a stall condition is detected.
 2. Themotor controller system of claim 1 wherein the voltage sensor senses RMSvoltage.
 3. The motor controller system of claim 1 wherein the voltagesensor senses instantaneous voltage for each winding of the motor anddetermines RMS motor voltage for each winding.
 4. The motor controllersystem of claim 3 wherein the control circuit detects a stall conditionusing an average of the RMS motor voltage for each winding.
 5. The motorcontroller system of claim 1 wherein the control circuit varies firingof the solid state switches to control motor starting current.
 6. Themotor controller system of claim 1 wherein the control circuit detects astall condition if a difference in the sensed voltage after a selectinterval is less than a select threshold.
 7. The motor controller systemof claim 6 wherein the select interval is about ten electrical cycles.8. A motor starter system comprising: solid state switches forconnection between an AC line and motor terminals for controllingapplication of AC power to the motor; a voltage sensor for sensing ACline voltage and motor terminal voltage; current sensors for sensingmotor current; and a control circuit for controlling operation of thesolid state switches, the control circuit limiting switch current duringa start mode and detecting a stall condition responsive to sensed ACline voltage and motor terminal voltage and selectively boosting motorcurrent during the start mode if a stall condition is detected.
 9. Themotor starter system of claim 8 wherein the voltage sensor senses RMSmotor voltage.
 10. The motor starter system of claim 8 wherein thevoltage sensor senses instantaneous voltage for each winding of themotor and determines RMS motor voltage for each winding.
 11. The motorstarter system of claim 10 wherein the control circuit detects a stallcondition using an average of the RMS motor voltage for each winding.12. The motor starter system of claim 8 wherein the control circuitvaries firing of the solid state switches to control motor startingcurrent.
 13. The motor starter system of claim 8 wherein the controlcircuit detects a stall condition if a difference in the sensed motorvoltage after a select interval is less than a select threshold.
 14. Themotor starter system of claim 13 wherein the select interval is aboutten electrical cycles.
 15. A motor starter system comprising: solidstate switch means for connection between an AC line and motor terminalsfor controlling application of AC power to the motor; voltage sensingmeans for sensing motor winding voltage; current sensing means forsensing motor current; and start control means operatively connected tothe voltage sensing means and the current sensing means for controllingoperation of the solid state switch means during a start mode, the startcontrol means limiting switch current during the start mode anddetecting a stall condition responsive to sensed motor winding voltageand selectively boosting motor current during the start mode if a stallcondition is detected.
 16. The motor starter system of claim 15 whereinthe voltage sensor sensing means senses instantaneous voltage for eachwinding of the motor and the start control means detects a stallcondition using average RMS motor voltage.
 17. The motor starter systemof claim 15 wherein the start control means varies firing of the solidstate switch means to control motor starting current.
 18. The motorstarter system of claim 15 wherein the start control means detects astall condition if a difference in the sensed motor voltage after aselect interval is less than a select threshold.
 19. The motor startersystem of claim 18 wherein the select interval is about ten electricalcycles.
 20. A method of detecting a stall condition during motorstarting comprising: providing solid state switches for connectionbetween an AC line and motor terminals for controlling application of ACpower to the motor; sensing motor voltage; sensing motor current;controlling operation of the solid state switches during a start mode tolimit switch current during the start mode; detecting a stall conditionresponsive to sensed motor voltage; and boosting motor current duringthe start mode if a stall condition is detected.
 21. The method of claim20 wherein sensing motor voltage comprises sensing instantaneous voltagefor each winding of the motor and detecting a stall condition isresponsive to RMS motor voltage.
 22. The method of claim 20 whereindetecting a stall condition comprise determining if a difference in thesensed motor voltage after a select interval is less than a selectthreshold.
 23. The method of claim 22 wherein the select interval isabout ten electrical cycles.
 24. The method of claim 22 wherein theselect interval is adjustable from about 1 to 1000 msec.